feat(ha-commit-durability): cap commit self-contribution at durable_lsn + durable-advance commit wake (task 2.2)

src/include/persist.h

@@ -122,6 +122,19 @@ struct persist_s {
     pthread_mutex_t fsync_mutex;   /* guards the fsync timer wait */
     pthread_cond_t  fsync_cond;    /* signaled by persist_stop for prompt join */
 
+    /* Durable-advance wake hook (ha-commit-durability task 2.2).
+     * Persistence owns this slot; the commit engine REGISTERS a callback
+     * (ctx = the commit_engine_t) via persist_set_durable_cb. The flusher
+     * invokes it AFTER advancing durable_lsn and AFTER releasing p->lock,
+     * so a COMMIT_MODE_WAIT caller is woken promptly when its entry becomes
+     * durable locally — without it, a commit whose standby ACKs all arrived
+     * before the local fdatasync would stall until the WAIT timeout. The
+     * dependency direction is preserved (commit depends on persist, never
+     * the reverse): persist only stores+calls an opaque function pointer.
+     * NULL until registered (every flush is then a no-op wake). */
+    void (*on_durable_advance)(void* ctx);
+    void*  durable_cb_ctx;
+
     /* Checkpoint (stubs for now — wired in Task 10) */
     atomic_int      checkpoint_requested;
     pthread_cond_t  checkpoint_done;
@@ -206,6 +219,16 @@ int persist_flush(persist_t* p);
  * any thread. Returns 0 when nothing is yet durable. */
 uint64_t persist_durable_lsn(persist_t* p);
 
+/* Register a durable-advance wake callback (ha-commit-durability task 2.2).
+ * The group-commit flusher calls fn(ctx) after it advances durable_lsn and
+ * after releasing p->lock, so a consumer (the commit engine) can re-evaluate
+ * its commit majority and wake any COMMIT_MODE_WAIT caller whose entry just
+ * became durable locally. fn MUST NOT take p->lock (it runs after release, on
+ * the flusher thread) — the commit engine's callback takes only ce->lock,
+ * keeping the lock order p->lock → ce->lock cycle-free. Pass fn = NULL to
+ * unregister. Safe to call before persist_start (no flusher running yet). */
+void persist_set_durable_cb(persist_t* p, void (*fn)(void* ctx), void* ctx);
+
 #ifdef XCE_TESTING
 /* ============================================================
  * Durability-verification test seam (ha-commit-durability Req 3.2, 3.3)